SSRS Report Server: Why do borders look so weird?

Question

I'm building a report with mutiple tables using SSRS and I really made an effort to make it look clean, so I positioned all the tables so the are nicely aligned. In the reportbuilder itself, it looks good:

But when I run the report, it looks like this:

Finally, when I look at the report in the frontend, it's even more off:

Some borders are missing, some are thicker than they should. I tried adjusting the bordersettings, but nothing seems to work, really annoying :)

As an experiment, I copied one of the tables, and this was the result. The lower table is the original table. I copied this and pasted it above. The black border is thicker in one table and the bottom border is thicker in the other table:

I hope there is someone out there who knows how to fix this!